This invention relates to reproduction of a range of gray levels as required by graphics arts applications. A common method of obtaining such gray levels involves the use of a halftone screen which causes the printing of dots having various sizes depending upon the gray level of the area represented by the dots. In facsimile applications wherein a halftone screen may not be employed, other gray level reproduction techniques have been used as described for example in Ranger, U.S. Pat. No. 1,848,840, Ernst U.S. Pat. No. 3,197,558, Young U.S. Pat. No. 3,294,896, McConnell U.S. Pat. No. 3,629,496, or in Klensch U.S. Pat. No. 3,580,995.
Another type of gray level reproduction to which this invention particularly relates is shown in Behane et al. U.S. Pat. No. 3,604,846. In the Behane patent gray level reproduction is accomplished by placing various numbers of uniformly sized dots within the cells of two-dimensional matrix. As described in detail in the Behane patent, an original representation such as a photographic negative or film positive is scanned optically, and density variations are reproduced electrically, converted to digital information, and recorded. The recorded information is then processed in a computer; the output produced being digital information representing a series of dots as required to construct a matrix print of appropriate optical density. The digital output information may be used to control any convenient ON/OFF marking device but preferably there is employed a binarily switched ink jet recorder as also disclosed in the Behane patent.
A significant limitation of an image reproduction system as disclosed in the Behane patent is the limited number of gray levels which may be obtained from a matrix of given size. For instance, a 3 by 3 matrix can only produce a total of only 10 gray levels, although most of the gray levels may be represented by several different dot arrangements. In those applications wherein more gray levels require reproduction it is necessary to increase the size of the matrix. Thus by increasing the matrix size to an 8 by 8 arrangement it becomes possible to reproduce a total of 65 gray levels running from white to black. Unfortunately, however, the matrix which is used for gray level reproduction is also the basic resolution element of the system, so that as the matrix increases to accommodate additional gray levels, the system resolution decreases. For instance the above mentioned 3 by 3 matrix which can accommodate 10 gray levels has a resolution of about 48 lines per inch when using a spacing of 0.0035 inches between matrix cells. On the other hand a 4 by 4 matrix of similar cell size gives 17 gray levels, but the resolution drops to about 36 lines per inch. Obviously this situation can be improved by decreasing the size of the matrix cells, but this also requires a decrease in the size of the printed dots. At this time it is not practical to produce a jet drop recorder which will print dots of a size very much smaller than about 0.0035 inches in diameter. Accordingly there has existed a requirement for a system which can reproduce a large number of gray levels in matrix form, but without undue degradation in resolution.